This invention relates to liquid crystal film laminates. More particularly, the invention relates to a liquid crystal film laminate which embodies an indicator component and a supportive and protective component removable therefrom.
Cholesteric liquid crystalline phase materials, also referred to as cholesteric liquid crystals and herein referred to for convenience at times simply as "liquid crystals," are a class of compounds that display a cholesteric mesophase or are in a cholesteric mesomorphic state within certain temperature limits. The compounds, their properties and their uses are well known, and they are described in our U.S. Pat. No. 4,161,557 and in the references cited therein, among others. As described in the patent, the liquid crystals in the cholesteric mesophase reflect light, the dominant wave length of which is affected by stimuli applied to the liquid crystals, such as heat, physical stress, electricity, magnetism, and radiation, which are types of applied energy, the presence of other materials, which may be present as contaminants, and materials present in the environment and which act upon the liquid crystals.
An important property of the individual cholesteric liquid crystalline phase materials and of mixtures of different materials is that, all other conditions being equal, they always display the same color at a specific temperature. The characteristic color is changed by the application of one of the above-described stimuli, furnishing a direct, visible response which may be utilized in a detector system. A stimulus either can interact directly with the liquid crystals, or can be converted into a stimulus which does interact. The color responses to stimuli are utilized in systems for measuring and/or detecting specific temperatures and temperature changes, detecting radiation, measuring stress, monitoring the presence of gases and vapors, and other purposes.
The temperature sensitivity of the liquid crystals finds important use in thermometers and thermographic systems. In general, when viewed against a black absorptive background color, the liquid crystals change in color from red to green to blue to violet with increasing temperature in the mesophase temperature range, and the reverse color change takes place with decreasing temperature therein. Above and below the mesophase temperature range, the liquid crystals assume the color of the background.
Liquid crystals must be protected or shielded from the atmosphere unless they are used only for a relatively short period of time. Exposure to the atmosphere results in oxidation, causing the liquid crystals to deteriorate. Foreign particles from the atmosphere provide sites for crystallization and alter the delicately balanced mesomorphic state of the liquid crystals. Ultraviolet radiation may cause deterioration of the liquid crystals.
For protection against such exposure, protection against physical contacts, cleanliness, convenience in handling, and other reasons, liquid crystals in the past have been enclosed in various ways. In particular, liquid crystal-containing coatings, layers, films, sheets and the like have been provided, wherein the liquid crystals are protected by encapsulation or by dispersion in plastic materials. The liquid crystals have been encapsulated in gelatin-gum arabic, polyvinyl alcohol, zein, or other substances, such as disclosed in U.S. Pat. Nos. 3,585,381 and 3,697,297. Discrete naked aggregates of the liquid crystals have been dispersed in a substantially continuous solid polymeric matrix by drying an emulsion of the liquid crystals in an aqueous solution of a polymer such as polyvinyl alcohol or a polyacrylate, as disclosed in British Pat. No. 1,161,039, published Aug. 13, 1969, and U.S. Pat. No. 3,600,060, and by forming an organic solvent solution of a polymer and the liquid crystals, and removing the solvent to form a film or layer, suitable polymers including polyvinyl butyral, acrylic resin, styrene resins, polyester resins, epoxy resins, polyvinyl chloride, polyvinyl acetate, and polycarbonate, as disclosed in U.S. Pat. No. 3,620,889, and polyurethane, as disclosed in U.S. Pat. No. 3,872,050. The aforementioned U.S. Pat. No. 4,161,557 discloses an improvement in the manufacture of a polyvinyl butyral film having liquid crystals dispersed therein, which results in accentuated intensity and contrast of visible light waves reflected by the liquid crystals in the film.
In practical application, liquid crystals although protected by dispersion of naked aggregates thereof in a film or layer of a film-forming polymer, or by encapsulation and additionally by dispersion of the liquid crystal-containing capsules in a film or layer of a film-forming polymer, have been supported and protected by an additional sheet or strip of plastic material, the sheet or strip being permanently affixed to the liquid crystal film or layer. An indicator article having the foregoing makeup is disclosed in U.S. Pat. No. 3,998,210. Additional articles of the foregoing makeup are described in U.S. Pat. No. 3,661,142 and in an unprinted publication of Dixon, Westervelt and Scala, entitled "Continuous Production of Laminated Cholesteric Liquid Crystal Films," which articles additionally include a protective film or layer of a film-forming polymer over the liquid crystal layer on the opposite side from the supporting sheet or strip.
The supporting sheets or strips employed in the past have been relatively thick and inflexible, causing the indicator articles to separate from uneven surfaces. The overall mass of the indicator articles has been excessive, resulting in diffuse coloration and reduced accuracy of temperature response. When a liquid crystal layer is to be viewed through a sheet, strip or film which protects an exposed surface of the layer, the clarity of the color response is reduced, i.e., the intensity of the reflected or scattered color is lowered and the color contrast is diminished, limiting the definition which may be achieved in making various studies and analyses. As a result, the prior articles have not been widely accepted for use in applications where they must be employed in intimate contact with uneven surfaces and/or where they must produce accurate, clear and well-delineated color responses to applied energy, in particular, in thermometric and/or thermographic applications on skin surfaces of the human body.